The present invention relates to liquid crystal displays used for information systems, such as word processors and notebook-type personal computers, video equipment of various kinds, video game machines, portable VCRs, digital cameras, etc. More particularly, the present invention relates to liquid crystal displays used indoors and outdoors, or in automobiles, air-planes, marine vessels, etc. where a variety of ambient light conditions occurs.
Conventionally, CRTs (Cathode Ray Tubes), EL (Electroluminescence) elements, PDPs (Plasma Display Panels), etc. have been put into practical use as displays of the light emissive type in which display contents can be overwritten electrically.
However, since this type of displays emit display light and use the same directly for the display, there arises a problem that their power consumption is quite large. Further since a light-emitting surface of the displays of this type serves as a display surface having high reflectance, if the displays of this type are used under the circumstances where ambient light is brighter than the luminance, for example, in direct sunlight, there always occurs a phenomenon known as xe2x80x9cwash-outxe2x80x9d in which the display light can not be observed.
On the other hand, liquid crystal displays have been put into practical use as color displays which display characters and/or images not by emitting the display light, but by adjusting an amount of transmitted light from a particular light source. These liquid crystal displays include a transmission type and a reflection type.
Of the two types, particularly popular are the liquid crystal displays of the transmission type which employ a light source called xe2x80x9cback lightxe2x80x9d at the back side, namely, behind the liquid crystal cell. Since the liquid crystal displays of the transmission type are advantageous in thinness and lightness, they have been used in diversified fields. On the other hand, the liquid crystal displays of the transmission type consume a large amount of power to keep the back light turned ON. Thus, regardless of the advantage that only a small amount of power is consumed to adjust transmittance of the liquid crystal, a relatively large amount of power is consumed as a whole.
However, the liquid crystal displays of the transmission type (that is, color liquid crystal displays of the transmission type) wash out less frequently compared with the displays of the light emissive type. This is because, in the color liquid crystal displays of the transmission type, the reflectance on the display surface of a color filter layer is reduced by the reflectance reducing technique using a black matrix.
Nevertheless, it becomes too difficult to observe the display light on the color liquid crystal displays of the transmission type when they are used under the circumstances where the ambient light is very strong and the display light is relatively weak. This problem can be eliminated by using brighter back light, but this solution raises another problem that the power consumption is further increased.
Unlike the displays of the light emissive type and liquid crystal displays of the transmission type, the liquid crystal displays of the reflection type show the display using the ambient light, thereby obtaining display light proportional to an amount of the ambient light. Thus, the liquid crystal displays of the reflection type are advantageous in a principle that they do not wash out, and when used in a very bright place in direct sunlight, for example, the display can be observed all the more sharply. Further, the liquid crystal displays of the reflection type do not use the back light for the display, and therefore, have another advantage that the power for keeping the back light turned ON can be saved. For the above reasons, the liquid crystal displays of the reflection type are particularly suitable as the devices for the outdoor use, such as portable information terminals, digital cameras, and portable video cameras.
However, since these conventional liquid crystal displays of the reflection type use the ambient light for the display, the display luminance largely depends on the surrounding environment, and when used under the circumstances where the ambient light is weak, there arises a problem that the display content can not be observed. Particularly, in case that a color filter is used for realizing the color display, the color filter absorbs the light and the display becomes darker. Thus, when used under these circumstances, the above problem becomes more apparent.
To eliminate the above problem, a lighting device called xe2x80x9cfront lightxe2x80x9d has been developed as an auxiliary light, so that the liquid crystal displays of the reflection type can be used under the circumstances where the ambient light is weak. Since the liquid crystal displays of the reflection type have a reflection layer behind the liquid crystal layer, they can not use the back light as do the liquid crystal displays of the transmission type. For this reason, the lighting device (front light) lights the liquid crystal displays of the reflection type from the front side, that is, from the display surface side.
On the other hand, liquid crystal displays, employing a transflective film which transmits a part of incident light and reflects the rest, have been put into practical use as the liquid crystal displays which can be used under the circumstances where the ambient light is weak while maintaining the advantages of the liquid crystal displays of the reflection type. The liquid crystal displays using both the transmitted light and reflected light are generally referred to as the liquid crystal displays of the transflective type.
For example, Japanese Laid-open Patent Application No. 218483/1984 (Tokukaisho No. 59-21843) (Japanese Patent Application No. 92885/1983 (Tokugansho No. 58-92885)) discloses a liquid crystal display of the transflective type which modulates the brightness by the TN (Twisted Nematic) mode, STN (Super-Twisted Nematic) mode, etc., which are known as the liquid crystal display modes for modulating the luminance of the transmitted light. Also, Japanese Laid-open Patent Application No. 318929/1995 (Tokukaihei No. 7-318929) discloses a liquid crystal display of the transflective type, in which a transflective film is provided in close proximity to the liquid crystal layer. Further, Japanese Laid-open Patent Application No. 160878/1994 (Tokukaihei No. 6-160878) (U.S. Pat. Nos. 5,598,285 and 5,737,051) discloses a liquid crystal display of the transmission type adopting the in-plane switching method as a technique for realizing a wider range of viewing angles. However, since the liquid crystal display of the transflective type disclosed in Japanese Patent Application No. 218483/1984 (Tokukaisho No. 59-218483) has the transflective film behind the liquid crystal cell seen from the viewer""s side, there occur the following problems (1) and (2).
(1) It is very difficult to set the brightness which affects a visibility of the display device. More specifically, when the brightness of the liquid crystal display of the transflective type is set adequately for the reflection display, the brightness is set high, so that it can be used under the circumstances where the ambient light is insufficient. However, if the brightness is set high by using a polarization plate having high transmittance in the TN method, for example, a contrast ratio, which is defined as a quotient obtained by dividing the brightness in the light display by the brightness in the dark display, becomes too low for the transmission display, thereby deteriorating the visibility. Conversely, when the brightness of the liquid crystal display of the transflective type is set adequately for the transmission display, it is preferable to set the brightness in such a manner as to raise the contrast ratio. However, in this case, the brightness becomes too low for the reflection display, thereby deteriorating the visibility as well.
(2) In the reflection display, since the display is observed by reflecting the light having passed through the liquid crystal layer sandwiched by the two substrates by the reflection film provided behind the liquid crystal cell, there occurs parallax (double image) and the resolution deteriorates, thereby making high-resolution display very difficult.
Also, in the liquid crystal display of the transflective type disclosed in Japanese Laid-open Patent Application No. 318929/1995 (Tokukaihei No. 7-318929), since the transflective film is used as the reflection film, there arises another problem that there is no optical design such that can be suitable for both the reflection display section and transmission display section.
Further, although the in-plane switching method disclosed in Japanese Laid-open Patent Application No. 160878/1994 (Tokukaihei No. 6-160878) is employed in the liquid crystal displays of the transmission type, the director configuration of the liquid crystal on the comb-shaped electrode does not contribute to the display. This is not because, in most cases, the electrode lines are made of metal that does not transmit light, but because the director configuration of the liquid crystal is not changed sufficiently for the transmission display.
Thus, to eliminate the above problems, the inventors of the present invention tried to apply the display method capable of eliminating the parallax and employed in the liquid crystal displays of the reflection type to the liquid crystal displays of the transflective type. More specifically, the inventors conducted an assiduous study by applying the two following methods to the transflective display:
(a) the GH (Guest-Host) method for filling liquid crystal composition blended with a dichroic dye into the liquid crystal layer; and
(b) the reflection type liquid crystal display method using a single polarization plate (hereinafter, referred to as the single polarization plate method).
To apply the above two display methods (a) and (b) which eliminate the parallax to the liquid crystal displays of the transflective type, the reflection layer is provided to touch or almost touch the liquid crystal layer, and a transmission opening is made through the reflection layer to use the transmitted light for the display in addition to the reflected light.
Then, the study revealed the following problems. In case of (a) GH method, when a concentration of the dichroic dye blended with the liquid crystal composition is adjusted adequately for the reflection display, the brightness is sufficiently high but the contrast ratio becomes too low in the transmission display section, thereby failing to obtain satisfactory display. On the other hand, when a concentration of the dichroic dye blended with the liquid crystal composition is adjusted adequately for the transmission display, the contrast ratio is sufficiently high in the transmission display section, but the brightness becomes too low in the reflection display section, thereby failing to obtain satisfactory display.
Also, in case of (b) single polarization plate method, the director configuration of the liquid crystal and a thickness of the liquid crystal layer which determine the optical characteristics, a voltage applied to the liquid crystal for driving the same, etc. are set adequately for either the reflection display section or the transmission display realized by additionally providing a polarization plate or the like behind the display surface (double polarization plate method).
Firstly, the display in the transmission display section when the thickness of the liquid crystal layer is set adequately for the reflection display will be explained. In this case, an amount of change in the polarization state caused when the director configuration of the liquid crystal layer is changed by an external field, such as an electric field, is about a strength such that can realize a satisfactory contrast ratio when incident light from the front, that is, from the display surface side, passes through the liquid crystal layer and exits to the display surface side by passing through the liquid crystal layer again. However, when set in this manner, an amount of the change of the polarization state of the light having passed through the liquid crystal layer is not sufficient in the transmission display section. Thus, even if the polarization plate used for the transmission display alone is provided behind the liquid crystal cell seen from the viewer""s side in addition to the polarization plate used for the reflection display and provided to the viewer""s side of the liquid crystal cell, that is, the display surface side, the display in the transmission display section is not satisfactory. In other words, when the director configurations (thickness of the liquid crystal layer, director configuration of the liquid crystal, etc.) of the liquid crystal layer are set to be suitable for the reflection display, in the transmission display section, either the brightness is not sufficient or even if the brightness is sufficient, the transmittance does not decrease in the dark display, thereby failing to attain a sufficient contrast ratio for the display.
To be more specific, in case of the reflection display, the director configuration of the liquid crystal in the liquid crystal layer is controlled by means of a voltage applied to the liquid crystal layer to impart a phase difference of about xc2xc wavelength to the light passing through the liquid crystal layer only once. When the transmission display is shown with the voltage modulation such that imparts a xc2xc wavelength phase modulation to the light passing through the liquid crystal layer set in such a manner as to impart the above-specified phase difference to the light passing through the same, if the transmittance of the transmission display section for the dark display is lowered sufficiently, about half the luminance of the light is absorbed by the polarization plate at the light outgoing side when the transmission display section shows the light display, thereby failing to obtain satisfactory light display. If optical elements, such as a polarization plate and a phase difference compensation plate, are provided to increase the brightness in the light display in the transmission display section, the brightness in the dark display in the transmission display section is increased to about half the brightness in the light display, and the resulting contrast ratio is not satisfactory for the display.
Next, the display in the reflection display section, in case that the director configurations of the liquid crystal layer are set to be suitable for the transmission display, will be explained. In case that the reflection display is shown when the liquid crystal layer is set adequately for the transmission display, the director configuration of the liquid crystal must be controlled by the voltage modulation in such a manner that the polarization state of the light passing through the liquid crystal layer only once is modulated between the two polarization states which are orthogonal each other. The two orthogonal polarization states include two linearly polarized light beams having oscillation planes intersecting at right angles, two circularly polarized light beams of right and left circularly polarization, or two elliptically polarized light beams having the same ellipticity whose major axis orientations intersect at right angles, thereby having opposite rotation directions in their respective photo-electric fields. To realize the modulation of the polarization state in any combination of the above two polarization states being orthogonal each other, a voltage must be modulated in such a manner that the liquid crystal layer imparts a phase difference of xc2xd wavelength to the light passing through the same. When the polarization state of the light is modulated by any combination of the two orthogonal polarization states in the above manner, satisfactory brightness and contrast ratio can be attained in the transmission display optionally, by the function of the polarization plate, with the help of the phase difference compensation plate.
However, when the liquid crystal layer is set to realize the above control, the reflectance in the reflection display is changed from the light display to the dark display and to the light display again while the transmittance in the transmission display is changed once from the light display to the dark display. Thus, the same display, that is, either the light or dark display, can not be realized simultaneously in the reflection display section and transmission display section by the same liquid crystal alignment changing means (for example, the thickness of the liquid crystal layer is equal, the initial director configuration is identical, and the driving voltage is equal). The problems raised in the methods (a) and (b) are also raised with the liquid crystal display of the transflective type disclosed in aforementioned Japanese Laid-open Patent Application No. 318929/1995 (Tokukaihei No. 7-318929).
In addition, a pressure detecting input device (touch panel) superimposed on the liquid crystal display has light reflecting properties, thereby posing a problem that the visibility is deteriorated. This problem is particularly obvious in the liquid crystal displays of the reflection type.
Also, in general, a front light unit used to improve the visibility of the liquid crystal displays of the reflection type under the circumstances where the ambient light is weak has a planar light pipe structure. Thus, the display content is observed through this light pipe, and there arises a problem that the visibility is deteriorated.
The present invention is devised to solve the above problems, and it is therefore an object of the present invention to provide a liquid crystal display with excellent visibility, capable of showing high-resolution display while using both the reflected light and transmitted light for the display. It is another object of the present invention to provide a liquid crystal display with excellent visibility, capable of showing high-resolution color display while using both the reflected light and transmitted light for the display.
The inventors of the present invention continued an assiduous study to fulfill the above and other objects, and achieved the present invention when they discovered that the cause of the problems occurred in the conventional liquid crystal displays applying either the GH method or polarization plate method is that the director configuration of the liquid crystal layer is set identical in the transmission display section and reflection display section at the same time.
Here, the director configuration of the liquid crystal layer indicates not only the director defined as orientation of the liquid crystal molecules at a specific point in the liquid crystal layer, but also the variation of the director field with respect to the position along the normal axis of the liquid crystal layer.
To be more specific, to fulfill the above and other objects, a liquid crystal display of the present invention is a liquid crystal display furnished with a liquid crystal display element having a pair of substrates, to which alignment members are provided to their respective opposing surfaces, and a liquid crystal layer sandwiched by the pair of substrates, characterized in that:
alignment mechanism for providing at least two different director configurations simultaneously on different arbitrary regions used for display in the liquid crystal layer is provided;
a reflecting member is provided to at least one of the different arbitrary regions showing different director configurations; and
the different arbitrary regions showing different director configurations are used for a reflection display section for showing reflection display and a transmission display section for showing transmission display, respectively.
According to the above arrangement, the director configuration of the liquid crystal can be different simultaneously. Thus, for example, an amplitude of modulation in an opti-physical quantity, such as an amount of absorbed light (absorbance) in case that a light absorber like a dichroic dye is used for the display, and a phase difference in case that optical anisotropy is used for the display, can be changed separately in each region having a different director configuration of the liquid crystal. Thus, according to the above arrangement, the transmittance or reflectance based on an amplitude of modulation in an opti-physical quantity in response to the director configuration of the liquid crystal layer can be obtained, thereby making it possible to set the optical parameters of the transmission display section and those of the reflection display section independently. Consequently, according to the above arrangement, it has become possible to provide a liquid crystal display of the transflective type with excellent visibility, capable of showing high-resolution display while using both the reflected light and transmitted light for the display.
Also, to fulfill the above and other objects, a liquid crystal display of the present invention is a liquid crystal display furnished with a liquid crystal display element having a pair of substrates, to which alignment members are provided to their respective opposing surfaces, and a liquid crystal layer sandwiched by the pair of substrates, characterized in that:
a region used for display in the liquid crystal layer is composed of regions having at least two different thicknesses of the liquid crystal layer;
the regions having at least two different thicknesses are used for a reflection display section and a transmission display section, respectively;
a reflecting member is provided at least to the reflection display section; and
the thickness of the liquid crystal layer is thinner in the reflection display section than in the transmission display section.
According to the above arrangement, the transmittance or reflectance based on an amplitude of modulation in an opti-physical quantity in the regions having different thicknesses of the liquid crystal layer can be obtained, thereby making it possible to set the transmission display section and reflection display section independently. Thus, according to the above arrangement, it has become possible to provide a liquid crystal display of the transflective type with excellent visibility, capable of showing high-resolution display while using both the reflected light and transmitted light for the display.
According to the present invention, satisfactory display can be shown on both the reflection display section and transmission display section by providing the above arrangement to the liquid crystal display. However, there is an optimal ratio of the reflection display section to the transmission display section for showing satisfactory display, and this optimal ratio varies depending on whether color display or monochrome display is desired, or whether the display is shown mainly by the reflection display or transmission display.
In the liquid crystal display of the present invention, in case that both the reflection display section and the transmission display section show color display, it is preferable that an area of the reflection display section accounts for 30% or above and 90% and less of a total of areas of the reflection display section and the transmission display section.
When the color display is shown on the liquid crystal display of the present invention in the above manner, besides the liquid crystal layer, design of the color filter layer, which plays an important role in color reproduction, is critical. According to the study of the inventors of the present invention, the liquid crystal display of the transflective type will be used in typical two styles.
One is a style that mainly shows the transmission display in general use and uses the reflection display supplementarily, so that the wash-out can be prevented under the lighting environment where the ambient light is very strong, and therefore, can be used extensively in diversified lighting environments compared with the displays of the luminous type or the liquid crystal displays of the transmission type. The other is a style that mainly shows the reflection display in general use by exploiting the advantages of the reflection display that the power consumption is small and the lighting device known as the back light is turned ON only when used under the circumstances where the lighting is weak. Hence, like in the former style, this style can be used extensively in diversified lighting environments.
In the former style (the style showing the transmission display mainly), by providing a color filter having a transmission color at least in the transmission display section of the regions making up the region of each pixel in at least one of the pair of substrates, it has become possible to provide a liquid crystal display with excellent visibility, capable of showing high-resolution color display while using both the reflected light and transmitted light for the display.
When the color display is shown in the above manner, it is effective if the color filter having a transmission color is provided at least to the transmission display section in each pixel, and in the reflection display section, either no color film is used or a color filter having the same brightness as the brightness of the color filter provided to the transmission display section or a color filter having a transmission color brighter than the brightness in the color filter provided to the transmission display section, is provided at least partially.
In the latter style (the style showing the reflection display mainly), by providing a color filter having a transmission color at least in the reflection display section of the regions making up the display region of each pixel in at least one of the pair of substrates, it has become possible to provide a liquid crystal display with excellent visibility, capable of showing high-resolution color display while using both the reflected light and transmitted light for the display.
When the color display is shown in the above manner, it is effective if the color filter having a transmission color is provided to at least the reflection display section in each pixel, and in the transmission display section, either no color film is used or a color filter having chroma as good as the chroma of the color filter provided to the reflection display section or a color filter having a transmission color with better chroma than the chroma of the color filter provided to the reflection display section, is provided at least partially.
According to the above arrangement, it has become possible to provide a liquid crystal display with excellent visibility, capable of showing a high-resolution color display while using both the reflected light and transmitted light for the display.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.